Older Male with Red Nodule on Arm

Specimen Type:

Skin

History:

The patient is 84-year-old man with a red nodule of one year duration on the arm. He has no systemic symptoms or other abnormal physical findings related to this nodule. The biopsy was from the nodule.

Pathologic Features:

The specimen has proliferation of small, bland spindle-shaped cells throughout the reticular dermis sparing the adnexal structures and adipose tissue. No atypical cells or mitotic figures were seen. The overlying epidermis was normal for the anatomic site. No significant inflammatory infiltrate was present. The cells are positive for SMA and negative for S-100 protein.

Differential Diagnosis:

Fibromatosis

Dermatofibroma

Dermatofibrosarcoma protuberans

Dermatomyofibroma

Diagnosis:

Dermatomyofibroma

Dermatomyofibroma is a rare, benign, dermal tumor of fibroblastic/myofibroblastic origin. It occurs most frequently on the shoulder and arm and tends to be in young adults, but has been reported in children. The primary differential is between a dermatofibroma and a dermatofibrosarcoma protuberans. The histologic features and immunoperoxidase staining help in this differentiation.